U.S. Pat. No. 4,289,812 discloses a method of preparing aqueous resin dispersions in which an epoxy resin is reacted with phosphoric acid and water, the resulting .beta.-hydroxy phosphomonoester groups ##STR1## are salified with an amine--ordinarily in excess over the stoichiometric requirement; water is added and the reaction medium is distilled out. Unless the free amine is relatively non-volatile, enough of it may co-distil so that the pH of the final dispersion will drop to such a low level that the resin particles will aggregate and/or deposit on the vessel surfaces, etc. On the other hand, it is also highly desirable not to have a higher final content of free amine than is really essential to the stability of the dispersion.
When the patented process is practiced on a small scale, enough excess amine can be added in the salification step to compensate for the amine removed by co-distillation and satisfactory solvent removal can be attained, without overheating of the dispersion, by transfer of the requisite heat through the walls of the distillation vessel ("kettle" or "pot"). However, when this was attempted on a pilot plant scale, serious degradation of the resin properties occurred; the high .DELTA.T required to achieve adequate heating of the main bulk of the dispersion at practicable stirring rates resulted in overheating adjacent to the kettle wall and consequent excessive amine removal (partially due to a higher degree of dissociation of the amine from the p--OH's in the salified ester groups).